In automatic transmissions known from prior practice, for supplying oil to the hydraulic system of the transmission and transmitting lubrication a mechanical pump device integrated inside the transmission device is usually provided in each case. The oil pump devices are each operated at the rotational speed of the transmission input, which usually corresponds to the speed of an internal combustion engine. In such a case the mechanical oil pump devices are each optimized to the speed range of the internal combustion engine, which preferably corresponds to 600 to 7000 revolutions per minute.
Transmission devices of vehicle hybrid drive trains, which in addition to an internal combustion engine also comprise an electric machine for driving the vehicle, are subjected to loads during the operation of the vehicle even when the internal combustion engine is switched off. For that reason, to avoid damage and to keep the transmission device ready for operation, the transmission devices have even then to be supplied with hydraulic fluid. To fulfill that requirement, when the internal combustion engine is switched off the transmission devices are often supplied with the desired hydraulic fluid volume flow by means of an electric auxiliary pump. Then, even when the internal combustion engine is switched off and the vehicle is stationary a gear can be engaged in the area of the transmission device, a parking lock can be disengaged, particularly in a transmission device designed with an E-mechanism, a gear engaged in the transmission device can be maintained by continuing the oil supply and compensating leakage flows, and a starting process of a vehicle can be carried out to the extent desired.
Moreover, by virtue of the electric auxiliary pumps gearshifts can also be carried out during electric drive operating conditions of vehicles at low rotational speeds of the transmission input, driving torques during traction or thrust operation of the hybrid drive trains can be produced, and the internal combustion engine can be started up by the electric machine.
Unfortunately, such auxiliary electric pumps give rise to high manufacturing costs and in each case take up substantial fitting space and increase the inherent weight of vehicle hybrid drive trains, all of which, however, are undesirable.